Light deflecting apparatus



June 3, 1969 W, BARKQW ET'AL 3,447,853

LIGHT DEFLECT ING APPARATUS Vsheet ors Filed July 30, 1965 Irfan/MV June 3, 1969 w. H. BARKow ETAL 3,447,853

' LIGHT DEFLECTING APPARATUS Filed July so, 196e sheet 3 of s Y INVENTOR Um/m /S/./mwa/ ima/wy United States Patent Office Patented June 3, 1969 3,447,853 LIGHT DEFLECTING APPARATUS William`Henry Barkow, Pennsauken, and Daniel Brasen, Hightstown, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed July 30, 1965, Ser. No. 476,000 Int. Cl. G02b 17/06 U.S. Cl. 350-7 8 Claims ABSTRACT OF THE DISCLOSURE Apparatus in which a first lens collects a deflected beam of light and focuses it to a d iffrctign limited spot and in which a second lens is positioned where the first lens reduces the deflection substantially to zero for focusing the diffraction limited spot to a reduced size.

This invention relates to light deecting apparatus and, more particularly, to such apparatus for use in high resolution micro-image scanning systems.

One system for scanning a line with a diffraction limited spot of light is shown in the prior art drawing of FIG- URE l. A light source .110 supplies a parallel beam of light 120, of a diameter equal to 2a. The beam strikes a spherical mirror 130, and is focused to a diffraction limited spot of light, which represents the minimum spot size obtainable, at point A on its focal plane 140, a focal length distance f away. The diameter of this spot, measured to the center of the first dark ring of the resulting diffraction pattern, is given by the expression where A represents the wavelength of the incident light and sin a can be approximated by the fraction a/f. As the mirror 130 is rotated through an angle by a driving source (not shown, the diffraction limited spot is made to scan a length of line, l, along the focal plane 140. Assuming that the rotation of the mirror 130 from its quiescent condition is such that the angular displacement of the reflected beam is the same on either side of its initial position (indicated by the angle 'y in FIGURE l), then the length of the line scanned, l, equals 2f tan /2. The number of beam diameters in the line is therefore equal to 2a tan /2 As is readily apparent, this beam diameter resolution, l/d, is independent of the focal length, f, of the spherical mirror 130.

Such a system as that just described is not generally suitable for use in line micro-scanning operations where high resolution-such as that existing in present day television broadcast practice-is required. It can be shown, for example, that in order to scan a micro-image one millimeter on a side with a 2 micron diameter spot of light-so as to obtain a resolution of 500 beam diameters-a spherical mirror having a focal length of .13 inch would be required if the incident beam of light were supplied from a helium-neon gas laser of 6,328 A. wavelength and of a diameter equal to .l inch. Such a short focal length mirror is not easily obtainable using todays manufacturing techniques, however. And it is that fact, therefore, which militates against using this sort of arrangement in the micro-image scanning environment. But even if such a mirror could be obtained on a commercial basis, the placing of the micro-image to be scanned at the focal length of the mirror, only .13 inch away from its reflecting surface, would undesirably block a significant portion of the supplied light from ever reaching the mirror to begin with. i

A second conventional system for scanning a line with a diffraction limited spot uses a rotating plane mirror to deflect the beam and a distant'lens to focus it to a spot. But this arrangement, also, is not suitable for microimage scanning usage; a .13 inch focal length would still be required for the lens, which in addition, would have to have an F number several times smaller than unity if it were to be sufficiently large in diameter to gather in substantially all the light deflected by the mirror. As almost goes without saying, the expense of manufacturing l such a lens would more than offset any benefits which might be derived by using it in such an image scanning environment. Although more practical F number lenses could be used by decreasing the lens diameter and by moving the lens in closer to the mirror at the same time, so as to maintain the amount of deflected light gathered in by it constant, such inward movement may also cause an interference with the light directed from the source to the mirror.

It is an object to the invention, therefore, to provide light deflecting apparatus for use in high resolution microimage scanning systems, which avoids the above-mentioned limitations and disadvantages of prior such apparatus.

As will become clear hereinafter, light deflecting ap- For a better understanding of the present invention,

together with further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIGURE 1 represents the prior art system of scanning a line with a diffraction limited spot of light as was referred to above;

FIGURE 2 represents one form of light deflecting apparatus constructed according to the invention; and

FIGURE 3 represents a modified form of light deflecting apparatus in accordance with the invention.

Description and operation of the light deflectng apparatus of FIGURE 2 Referring now more particularly to FIGURE 2 of the drawings, that ligure illustrates how the light dellecting apparatus of the present invention might be used in line-micro-scanning. The apparatus shown includes a light reflective surface 200 and, also, driving means 205 for rotating that surface. Such a combination may comprise the-we1l known electro-mechanical or dArsonval galvanometer type arrangement of using a mirror coupled to a coil suspended between the poles of a U-shaped magnet. Alternatively, it may comprise a tuning fork device, with appropriate reflecting accessories attached to the tines of the fork. Where high speed scanning is an additional system requirement, the use of any one of the torsional arrangements described in pending application, Ser. No. 473,329, filed July 20, 1965, and entitled, Torsional Light vDefiector, for this combination provides highly satisfactory results.

The light dellecting apparatus also includes means for directing a beam of light and, more particularly, a parallel beam of light 210, towards the light reflective surface 200 (hereafter referred to as the plane mirror 200, or simply as the mirror 200). Such means may comprise a laser or any source of.collimated light 215. In one form of this embodiment, @helium-neon gas laser having a power output of 100 microwatts, single Imode, at a wavelength of 6,328 A. was used as the light source 215. The diameter of the light beam 210 supplied by this gas laser, indicated as 2a in FIGURE 2, is of the order of .1 inch.

Assuming that the initial position of the plane mirror 200 is as shown in FIGURE 2, then the parallel beam of the mirror 200 at an angle equal to 0, and will appear as indicated by the beam 220. Let it now be assumed that the driving means 205--the electro-mechanical arrangement, the tuning fork arrangement, or the torsional arrangement, for example-rotates the mirror 200 through an angle 1,0/2 on either side of its initial position. This rotation causes the light beam 220 to be deected through an equal angular displacement from its initial or rest position, clockwise and counter-clockwise alike. For simplicity of FIGURE 2 of the drawings-and also for simplic ity of FIGURE 3, a description of which follows subsequently--only angular displacements on one side are shown. In addition, only the central ray of the deflected beam is shown and for the same reason. Thus, the central ray 225 of the beam 220 when deflected by an amount tlf/2, is shown in its new position by the dashed line 230. As will be noted, it is an angle H-i-tl//Z away from the central ray of the beam 210 as supplied by the source 215.

The light deflecting -apparatus of this embodiment of the invention additionally includes a first lens 235 for reducing the linear deflection provided by any of the abovementioned rotating arrangements and for focusing the deflected beam (as represented by the central ray 230) to a diffraction limited spot of light. This lens 235 is a thin lens of the convex variety and is positioned a distance Y plus its focal length f1 away from the minor 200 as shown. The lens 235, in addition, is so arranged that the central ray of the reflected light beam 225, with mirror 200 in the initial position, passes through its optical axis. With such an arrangement, each deflected light beam is focused to a diffraction limited spot on the focal plane 240 of the lens 235, a focal length distance f1 behind the lens 235-at point B for the reflected beam 220 at point C for the deflected beam indicated by the ray 230. The micro-image to be scanned is, as will become presently clear, not located at this focal plane 240, however, as contra-distinct from the second type of prior art scanning system described previously. The diameter of this diffraction limited spot, therefore, may be many times larger than the size necessary for purposes of micro-scanning.4

This, in turn, permits a wide variety of focal length lenses to beused for the lens 2-35, all other things remaining equal. By adjusting the distance Y so that a reasonable lens diameter may be selected to gather in the deflected light, practical F number lenses may be used in the configuration. In the form of this embodiment alluded to above a 1.79 inch focal length F1.5 16 mm. camera lens was used as the lens 235.

The diameter of the diffraction limited spot focused onto the focal plane 240-either at the points B or C or at any other point thereoncan be determined, once again, from the general expression sin a With a 6,328 A. wavelength incident beam of .1 inch diameter light and with a 1.79 inch focal length lens, the diameter of this spot can be calculated to be 27.7 microns. Such a spot size is obviously too large to provide any resolution approximating that existent in present day television practice when used to scan a micro-image l millimeter on a side when that micro-image is placed at (=36.1 beam diameters) the focal plane 240 The light deecting apparatus of FIGURE 2 further includes a second lens 245 for focusing the diffraction limited spot produced on the focal plane 240 down to a diminished size suitable for high resolution micro-image scanning. This lens 245 is positioned a distance X plus the focal length f1 of the lens 235 away from that lens. More particularly, the lens 245 is positioned at that point where the lens 235 effectively reduces the linear deflec- `J:igprlnzidfztby-therpirror rotatingarrangemgntsgtgzem. light 210 supplied by the source 215 will be reflected by 10 "u ith such a coniiguratitfthimage of the center of the mirror 200 falls substantially on the lens 245. While the invention operates best when the lens 245 is placed pre cisely where the resulting deflection is zero (point D on the central ray 225), highly satisfactory results are still obtained with the lens 245 placed slightly ahead of that point or slightly behind it. For best results, the lens 245 is so arranged that the ray 225 also passes through its optical axis. In the embodiment of the invention referred to above, a microscope objective with a focal length of 8 millimeters and with a numerical aperture of .5 was used as the lens 245. An equivalent numerical aperture F1.1 camera lens may similarly be used for the lens 245.

As was Ipreviously mentioned, this lens 245 is primarily concerned with focusing the deflected beam to a spot size suitable for purposes of high resolution micro-image scanning. This spot size is given by the expression where f2 represents the focal length of the lens 245; x represents, once again, the wavelength of the incident light; and a'- represents the radius of the light beam at the lens 245. The expression for a can be determined from the following mathematical proof.

From simple geometry, it is seen that G L a f1 (l) Substituting the Newtonian form of the thin lens equation X Y=f1"' (2) (See Principles and Applications of Physics-O. Bluh,

and J. Elder, Interscience Publishers, New York, 1955,

p. 390) into the expression (l), it can be shown that With a 6,328 A. wavelength light beam of .1 inch diameter, a first lens 235 of a 1.79 inch focal length, a second lens 245 of an 8 millimeter focal length-.5 numerical aperture, and a lens separation of 27 centimeters the diameter of this spot can be shown to be approximately 1.8 microns. With the microimage position at the focal plane 250 and with such an arrangement, more than 500 beam diameters resolution can be obtained when the spot is made to scan the entire one millimeter length of the micro-image.

It will now be shown that this resolution is identical to that obtained with the prior art focusing arrangement described above. It will be remembered that that resolution was said to be equal to f (4) But 2/2=(Y+f1) tan Blf/2 (5) X Y=fr2 (2) Substituting (2) and (5) into (4) and cross-multiplying: Q=tft2+Xf0 tan M2 and 2 X 'lf 1 l1 5=f1 tan However, um 11 X (7) Substituting (2) and (6) into (7) and cross-multiplying:

f2Y tan M2 2 fr (8) Since 13/2 represents the linear displacement on one side of the optical axis only, the total displacement, i.e., the length of the line scanned, I3, at the focal plane 250, is given by two times the expression (8). The number of beam diameters in the scanned line la is therefore equal to:

(2]2Y tan 1,0/2) afl dzf1 1.22f2Y Thus, by providing equal angular rotations to the respective rellecting mirrors (4: in FIGURE l and 1p in FIG- URE 2), equal beam diameter resolution is obtainable with either configuration. The arrangement shown in FIGURE 2, however, permits reasonable, and therefore, economically practical, lenses to be used in auV arrangement wherein all of the light supplied by the source reaches the reflecting mirror.

It is to be noted in passing that this same beam diameter resolution also exists at the focal plane 240 of the lens 235, where the length of the scanned line l1=2f1 tan #//2 and the diameter of the diffraction limited spot The significance of this fact will become clear when the reader considers the information writing applications of the'invention described below.

and operation of the light deflecting apparatus of FIGURE 3 The light deflecting apparatus just described and shown in FIGURE 2 essentially represents a line micro-scanner. The. light deecting apparatus shown in' FIGURE 3, ou the. other hand, represents a raster micro-scanner, in which corresponding components carry the same reference numerals as in FIGURE 2, except preceded by the numeral 3.

Referring to FIGURE 3, a light rellector 360 and driving rotational means 355 are inserted in place of the lens 245 of FIGURE 2. The axis of rotation for this reflector 360 is arranged to be perpendicular to the axis of rotation for the mirror 300. The mirror 300 can be aligned to scan in a horizontal direction, for example, while the reflector 360 can be aligned to scan in a vertical direction. The rotational combination 355, 360 may also be of the electro-mechanical, tuning fork, or torsional type, just as in the apparatus of FIGURE 2.

Description Since the center of the mirror 300 is imaged at the position where the rellector 360 is placed, and since the reflector 360 is so arranged that the central ray from the lens 335 passes through its center, both the deflection centers of the mirror 300 and the reflector 360 will be superimposed. The vertical deflection provided by the reflector 360 will then also be superimposed on the horizontal deflection provided by the mirror 300, to'forrn a scanned raster. The scanning spot of light is then focused by a second lens 365, which performs a function similar to that provided by the lens 335 of FIGURE 2; namely, t0 reduce the overall linear deflection. At the point of zero deflection G, a third lens 370 is inserted to focus the scanning spot to a size suitable for high resolution micro-scanning purposes. This lens 370 therefore performs a function similar to that provided by the lens 245 of FIGURE 2. The scanned raster will then have at its focal plane 375 approximately the same resolution as the raster will have at the focal plane 380 of the lens 365, just as the resolution of the scanning line at the focal plane 250 of the lens 245 in FIGURE 2 equalled that at the focal plane 240 of the lens 235.

Such light detlecting apparatus as is constructed according to the present invention can be extremely effective in the reading of high density information from a microimage surface. When used as a reading device, it is desirable to include a photo-multiplier pickup tube in the scanning system to convert the light modulated by the microimage pattern into an electrical signal.

The light deflecting apparatusis perhaps more useful as a writing device, for writing micro-image information onto a surface. When using a laser as the light source, the power density of the focused spot with this arrangement can be made extremely higha one milliwatt laser beam focused down to a two micron spot, for example, will have a power density of approximately l5 kilowatts per square centimeter when the light transmission through the optical system is only The micro-image information may be Ilaid down on the surface by modulating the laser beam with electro-optical means or by scanning the beam through a pattern placed at one of the intermediate focal lplanes-at 380 in FIGURE 3, for example. By placing patterns at more than one focal plane and/or by modulating the beam with an electro-optical modulator, a number of multiple signals can be added together to form the lfinal image on the nal focal plane.

Other applications for the light deecting apparatus will also be apparent. In one example, by using four deflectors in an arrangement similar to the two dellector arrangement of FIGURE 3, a system can be developed for scanning a micro-image situated within a larger image area at the same time that the larger image area is scanned. These two scannings can be carried out at the same or different rates, whichever is desired. In another example, 'by placing the lens 370 of FIGURE 3 either before or after the point of zero de-ection, G, a xed amount of olfset can be established between the deflection centers of the mirror 300 and the reector 360. Such offset is desirable when the light detlecting apparatus is employed in the printing of shadow-mask screens for color kinescopes, another one of its many uses.

With a high energy laser beam as the light source, the deflecting apparatus of the present invention can additionally be used in milling and welding operations, as well as in other machine shop applications. With the advent ofA higher energy lasers than are presently commercially available, deflection apparatus using such lasers `and constructed according to the invention can, furthermore,

7 area with a spot of light whose diameter is less than that of an incident light beam.

What is claimed is:

1. Light defiecting apparatus comprising:

a light reflective surface;

first means for directing a beam of light towards said surface;

second means for controlling said surface to defiect said beam;

a first lens for reducing the linear defiection provided by said second means to said bearrkand fgtacusing s'fndbearnfamifratigrilimited Spcand a second lens positioned where said first lens reduces said deflection substantially to zero for focusing said diffraction limited spot to a diminishedsize.

2. Light defiecting apparatus according to claim 1 in which said second lens is positioned where said first lens reduces said deflection to zero.

3. Light detlecting apparatus according to claim 1 in which said first means directs a highly collimated beam of light towards said light refiective surface and in which said second means rotates said surface to deflect said beam.

4. Light deliecting apparatus according to claim 3 in which the focal length of said second lens is substantially smaller than the focal length of said first lens, for focusing vsaiddilractonalirnitedkgggttoma diminished size suitable for high resolution micro-scanning.

5. Light defiecting apparatus according to claim 4 in which said first lens and said second lens are so arranged that their optical axes coincide with one another.

6. Light deflecting apparatus according to claim 5 in which said first means directs a laser beam of light of 6,328 A. wavelength and .1 inch diameter towards a light refiective plane mirror and in which said first and second lenses are a 1.79 inch focal length, Fl.5 16 mm. camera lens and 8 mm. focal length. .5 numerical aperture microscope objective, respectively.

7. Light deliecting apparatus comprising:

a first light reflective mirror;

means for directing a beam of light towards said mirror;

first means for rotating said mirror in a first plane to deliect said beam in said plane;

a `first lens for reducing the linear defiection provided by said first rotating means to said beam and for focusing said beam to a first diffraction limited spot;

a second light reflective mirror positioned where said first lens reduces said linear deflection to zero;

second means for rotating said second mirror in a plane perpendicular to that of said first plane, to defiect the beam of light passing through said first lens in said perpendicular plane;

a second lens for reducing the linear deflection provided by said second rotating means to said passing lbeam and for focusing said beam to a second diffraction limited spot; and a third lens of substantially smaller focal length than that of said first and second lenses, positioned where said second lens reduces said linear deflection in said perpendicular plane to zero, for focusing said second diffraction limited spot to a diminished size suitable for high resolution scanning of a micro-image.

8. Light deflectng apparatus according to claim 7 in which the optical axes of said first and said second lenses are so arranged with respect to their associated mirrors that the deflection centers of said mirrors are superimposed one upon the other at said third lens.

References Cited UNITED STATES PATENTS 1,979,296 ll/ 1934 Sweeney 350-6 X 2,068,829 1/ 1937 Albada 350-8 X 2,881,657 4/ 1959 Wittel. 2,977,847 4/ 1961 Meyer-Arendt 350-8 3,062,965 11/1962 Sick 350-7 3,153,723 10/ 1964 Weiss 350-7 3,163,754 12/ 1964 Bigelow. 3,180,216 4/ 1965 Osterberg 331-945 X 3,311,843 3/1967 Friedl 331-945 3,324,478 6/ 1967 Jacobs S31-94.5 3,107,296 10/ 1963 Hine 350-96 3,192,824 7/ 1965 Rosenthal 350-7 3,281,712 10/ 1966 Koester 33--94.5 3,345,120 10/ 1967 Palmer 350-6 FOREIGN PATENTS 1,306,777 9/ 1962 France.

DAVID SCHONBERG, Primary Examiner. PAUL R. GILLIAM, Assistant Examiner.

U.S. Cl. X.R. S-45, 55 

